The recovery of mineral values from mineral-bearing ores may be achieved by either in situ leaching or surface leaching operations. In surface leaching, the mineral-containing ores are mined to the surface where they may be crushed and mixed. The ores are held in holding tanks where they are subsequently subjected to surface leaching operations to recover the mineral values therefrom. Surface leaching, such as heat leaching or various other types of mill leaching, usually comprises oxidation of the mineral values and the solubilization thereof.
In situ solution mining of mineral values is a known alternative to surface recovery processes particularly when the latter are not economically feasible. Conventionally, in in situ solution mining processes, the leaching solution is brought into contact with the subterranean deposit through a suitable injection system. The leaching solution or lixiviant may be an alkaline or acidic medium which solubilizes the mineral values as it traverses the ore body. Often, the mineral values in an ore body are subjected to an oxidation step in order to convert them to a soluble form. For example, the tetravalent uranium must be oxidized to its soluble hexavalent form for leaching. The pregnant lixiviant, whether resulting from surface or in situ leaching operations, is then withdrawn from the ore body and treated to recover mineral values therefrom by suitable techniques such as solvent extraction, direct precipitation or by absorption and elution employing an ion exchange resin. All too often, however, and in fact in the majority of cases, the leaching will result in the release of other mineral values than the desired mineral value into the leachate. These other mineral values may not be present in sufficient quantities to justify surface recovery. However they may be present in large enough quantities wherein the formation fluids have to be treated to reduce the concentration of such mineral values or contaminants to environmentally acceptable levels. Additionally, after the recovery of the desired mineral values, the formation fluids have to be treated to reduce the content of solubilized radionuclides and heavy metals to environmentally acceptable levels.
For example, recovery of uranium values from subterranean formations involves the usual methods of oxidation of insoluble tetravalent uranium into soluble uranyl complexes that may be drawn from the formation by leaching. The overall reaction in oxidative in situ leaching may be described as follows: EQU UO.sub.2 +[O]+3HCO.sub.3.sup.- .fwdarw.UO.sub.2 (CO.sub.3).sub.3.sup.-4 +H.sup.+ +H.sub.2 O
The use of oxygen, however, also solubilizes other minerals and radionuclides. Thus, when the leaching operations are terminated, restoration of the contaminants in the formation becomes necessary. For example, in New Mexico current environmental regulations restrict the amount of uranium permissible in formations after leaching to less than 0.1 ppm. Other radionuclides, such as radium and thorium for example, are subject to similar restrictions.
Accordingly, the present invention provides a process for reducing the contaminants level, including mineral and radionuclide contaminants, in the formation fluids after the formation has been subjected to oxidative in situ leaching to recover mineral values therefrom.